Free-interface diffusion techniques have been employed in a number of scientific applications, most notably in protein crystallization studies. In a free interface, mixing between different fluids occurs entirely as a result of diffusion. The creation of such free interfaces offer the advantage of gradual and non-directional mixing, avoiding asymmetries and steep concentration gradients associated with convective flow between the fluids.
However, certain technical difficulties have rendered conventional macroscopic free interfaces unsuitable for high throughput screening applications, and they are not currently widely used in the crystallographic community for several reasons.
First, conventional macroscopic fluidic interfaces are established by dispensing solutions into a narrow container; such as a capillary tube having a width of 100 μm or greater, or a deep well in a culture plate. FIGS. 30A-B show simplified cross-sectional views of the attempted formation of a macroscopic free-interface in a capillary tube 9300. The act of dispensing a second fluid 9302 into a first fluid 9304 may give rise to convective mixing and result in a poorly defined fluidic interface 9306.
Unfortunately, the fluids may not be sucked into a capillary serially to eliminate this problem. FIGS. 31A-B show the mixing, between a first solution 9400 and a second solution 9402 in a capillary tube 9404 that would result due to the parabolic velocity distribution of pressure driven convective Poiseuille flow, also resulting in a poorly defined fluidic interface 9406.
Moreover, the container for a macroscopic free-interface crystallization regime must have dimensions making them accessible to a pipette tip, needle, capillary or other g tool, and necessitating the use of relatively large (10-100 μl) fluid volumes.
In order to avoid unwanted convective mixing during macroscopic free interface diffusion experiments, considerable care must be exercised both during dispensing of the fluids and during the diffusion period. For this reason cumbersome protocols are often used to define a macroscopic free-interface. In one conventional approach, one fluid may be frozen or otherwise converted to solid phase prior to addition of the second fluid. In an alternative conventional approach, the one fluid containing the sample is converted into a solid through polymerization, trapping the sample within the polymer, which is then exposed to the second fluid. See generally Garcia-Ruiz et al., “Agarose as Crystallization Media for Proteins I: Transport Processes”, J. Crystal Growth 232, 165-172 (2001).
The problems of convective mixing associated with macroscopic free interface diffusion studies is compounded in that two fluids of differing density will mix by gravity induced convection if they are not stored at the proper orientation, additionally complicating the storage of reactions. This is shown in FIGS. 32A-C, wherein over time first solution 9500 having a density greater than the density of second solution 9502 merely sinks to form a static bottom layer 9504 that is not conducive to formation of a diffusion gradient along the length of a capillary tube.
Other approaches to the formation of free interfaces have focused upon microfluidic structures. For example, Kamholz et al., “Quantitative Analysis of Molecular Interaction in a Microfluidic Channel: the T-Sensor”, Anal. Chem. Vol. 71, No. 23 (1999), describe the formation of a diffusive interface between two flowing fluids inlet on sides of microfluidic T-shaped junction and then moving together along the stem. The dimensions of the microfluidic channels impose laminar flow on the fluids, such that convective mixing between them is eliminated and diffusion only occurs across the interface.
While the diffusion described by Kamholz et al. may be useful, it offers the distinct disadvantage of necessarily creating an elongated interface between the fluids, such that substantial volumes of the fluids are continuously exposed to the steep concentration gradient occurring at the interface between them. Such a steep gradient poses a number of disadvantages. In the context of a protein crystallization experimental protocol, one such disadvantage is the possibility of solvent shock and the precipitation of sample in non-crystalline form along the interface.
Accordingly, there is a need in the art for methods and structures for accomplishing diffusive introduction of material under highly controlled conditions that limit the exposure of volumes of solution to steep concentration gradients.